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Otto Pregizer Optik

 

The COATEST 7L apparatus

 
This instrument is the outcome of 50-year experience in optical metrology. I am a retired astronomer and I do not need building and selling instruments, but I care very much providing astronomers, my fellows, with quality control instruments that match the quality of modern telescopes.

 
History
The first IRIS was build 15 years ago for checking the quality of car windshields in Sweden. It was originally a scatterometer. Winters are hard up there and they use nailed tires so that at the end of winter, most windshields are like sand blasted. The governmental road security organization asked me to develop a handy instrument for checking the quality of windshield in the field. Soon, a reflectometer was added to the same instrument and it became quite sophisticated, including four wave bands as it was provided to major observatories. Today, this instrument looks to me like a dinosaur; with my accumulated experience and the many discussions I had with astronomers, I am able today to propose what I think is the best up to date instrument for observatories. The electronic is based on a TI DSP board and will be up to date for years. Light sources are SM LED's associated with fiber optics; LED's can be easily upgraded if new wave bands are demanded.

 

Specifications (provisory)

The COATEST 7L is a new instrument specially thought for accurate long term assesment of the mirrors coating.
It has an acceptance angle of 0.8° (half angle) and works at 8° incidence.

The seven Bands are chosen to be equidistant on the energy scale.

Wavelength (nm) Bandpass (nm)
365 10
405 15
465 20
530 30
630 30
770 50
970 40

The sources are modulated at a frequency around 5Khz and detected by the technique of digital lock in amplifier based on a DSP board.

Two identical detectors are used; one to measure the reference signal of the source, the other to measure the reflected signal. Both signals are digitaly detected and the ratio of the source signal to the reference signal is recorded and processed to yield the reflectivity value. Each color is lit and measured sequentially for one second.

The irradiance is conducted to the detectors by optical fiber and the two detectors are mounted on the same heat sink. This insures a long term absolute stability and total immunity against temperature variations even when the sources degrade or vary in intensity. This instrument is factory calibrated once for all. Only dirt or pollution on internal optical elements could change the response and necessitate recalibration.
  • Global absolute accuracy : 0.5% for R>50%
  • Repeatability: 0.1% for R>50%
The apparatus is small (80 mm x 60 mm x 120 mm high) and light (<500 gr) and in one piece (no wire; one hand operation).
It is positionned by a three point soft contact. Curved mirrors can be measured without correction down to a radius of + or – 1 m.
Internal memory keeps more than 200 measurements including date, time and temperature, sequential number and individual identification. Battery last for more than 500 measurements.
Talking with the apparatus via PC and USB2 connection.
Batteries are charged while connected to the USB port of the PC; independent charger is also provided.

 

Dust detector

The Coatest 7L can be equipped with a dust detector. The latter measures the scattered light in red light (630 nm), in green light (530 nm), and possibly in UV light (365 nm). It measures the integrated scattered light between 9° and 19°. This measurement yields a fair approximation for the relative value of the TIS for the local type of dust at the site. The dust index indicates the quantity of dust (in relative surface) covering the mirror.

 

VW calibration setup

Although the Coatest 7L does not need recalibration unless it has been polluted, we have designed a VW jig that fits on the instrument and yields the absolute values of reflectivity of a gauge in the seven bands; this gauge can then be used immediately after calibration to check the  Coatest 7L  and readjust its software parameters.

This jig is a precision instrument with very accurate motions of an auxiliary mirror in order that the beams in the V configuration has exactly the same path as the beam in the W configuration except for two reflections on the unknown gauge. This will be developed and explained in an Application Note to come.